The overall goal of this application is to understand the transformation of a motile growth cone into a sedentary synapse. We will use the Drosophila giant fiber (GF) system as a model to study the way in which a single axon recognizes the target area and makes a synapse with appropriate partners. Through targeted gene expression in the GF and its synaptic partners, we will explore the molecular aspects of this dynamic transformation. Our hypothesis is that semaphorin1a, which is well known for its role as a ligand during pathfinding, has a second role as a receptor during synaptogenesis and that it is critical to the delicate and dynamic molecular interactions that control the transition from growth cone to synapse. In order to better understand this new role for semaphorin1a, we will dissect the intracellular domain of semaphorin1a and determine its function during synapse formation. We will also examine putative downstream signals and will focus on enabled as a potential partner for semaphorin1a in stopping the growth cone. Finally, we will assay the dynamic regulation of proteins like semaphorin1a by the ubiquitin cascade. One of the first mutants isolated in the Drosophila giant fiber system was bendless, a defect in an E2 component of the ubiquitin cascade that causes a defect in the maturation of the giant synapse. We propose to revisit these early findings to assess the targets for the ubiquitin cascade and how they contribute to the dynamic processes that underlie the transition from growth cone to synapse.